DDR2 dual channel

I was thinking of adding some memory and i wanted your opinion. Here's what i want to do: I've installed two 512mb 533mhz sticks o dual channel, and wanted to buy another pair but with 677mhz. So, is it ok to i put the 667 ones on dual channel and the 533 pair just for extra storage?, i mean is this the way or is best to mantain the same frequency between modules?